1. Field of the Invention
The present invention relates to an apparatus for driving a flyback transformer (FBT) which supplies high voltage to a cathode-ray tube (CRT) in a video display appliance, particularly to an apparatus for driving an FBT which can prevent deterioration of the FBT efficiency caused by excessive horizontal frequency input.
2. Description of the Prior Art
FBT driving apparatus is generally used to generate high voltage for driving a CRT-equipped video display appliance. As illustrated in FIG. 1, a conventional FBT driving apparatus comprises a pulse width modulation (PWM) transforming section 21 for comparing an input horizontal sync signal fh with a feedback signal to output a circular wave pulse, a field effect transistor FET21 for receiving the output circular wave pulse from the PWM transforming section 21 to switch and provide a Vcc power supply, a high voltage driving section 22 for driving a switching transistor Q21 by synchronizing it with the input horizontal sync signal fh, a flyback transformer FBT21 for generating high voltage towards a secondary winding by forming a resonance circuit with a diode D22, capacitor C22 and an interposed primary winding coil L22, and a high voltage detecting section 23 for detecting the high voltage generated in the FBT21 and feeding back the detected high voltage to the PWM transforming section 21.
Here, R21, R22 refer to resistors; D21, D23, D24, D25, D26 refer to diodes; L21, L23, L24, L25, L26 refer to coils; and C21, C23 refer to capacitors.
The operation of the conventional FBT driving apparatus for video display appliances constructed above will now be explained.
Referring to FIG. 1, the PWM transforming section 21 compares the input horizontal sync signal fh with the input feedback voltage detected by the high voltage detecting section 23 to output a circular wave pulse according to the resultant voltage. The field effect transistor FET21 is driven by the circular wave pulse to switch the Vcc power supply. The switched Vcc power supply voltage is supplied to the primary winding of the FBT21.
The input horizontal sync signal fh is supplied to the high voltage driving section 22. The switching transistor Q21 is driven by the output of the high driving section 22 which has been synchronized with the input horizontal sync signal fh, thereby driving the FBT21.
In other words, the resonance circuit formed by the primary winding coil L22 and the diode D22 as well as the capacitor C22 connected thereto is driven, thereby inducing AC high voltage to the secondary winding. The secondary induced voltage is rectified by the secondary winding coils L23, L24, L25 and the diodes D24, D25, D26. The rectified voltage is then smoothed by the capacitor C23 and supplied to the following circuits which require high voltage.
The high voltage supplied through the above process is level-detected by the high voltage detecting section 23 comprising the resistors R21, R22. The detected voltage is supplied to the PWM transforming section 21 as a feedback signal to control the high voltage supplied from the FBT21.
In the conventional FBT driving apparatus for video display appliances constructed above, the PWM transforming section 21 and high voltage driving section 22 are synchronized with the input horizontal sync signal fh. Thus, the operation of the FBT21 is controlled according to the horizontal frequency input.
If the frequency of the input horizontal sync signal fh elevates, B+ voltage also elevates, thereby resulting in heat loss of the diode D21 and coil L21 that rectify high horizontal frequency. Particularly, the high voltage diodes D23, D24, D25, D26 provided on the secondary winding of the FBT21 should rectify tens of kilo volts. Accordingly, if the horizontal frequency elevates higher than 100 khz, ordinary rectifying diodes cannot be used. Further, the efficiency of the secondary winding coils L23, L24, L25, L26 of the FBT21 drastically decreases, thereby resulting in a considerable amount of electric current leakage.